This application claims the benefit of co-pending German Patent Application No. 199 54 577.4-22 entitled xe2x80x9cLiftzylindereinheit fxc3xcr eine Hebebxc3xchnexe2x80x9d, filed on Nov. 12, 1999.
The present invention generally relates to a lifting cylinder unit for a lifting platform. Such a lifting cylinder unitxe2x80x94together with other structurally identical lifting cylinder unitsxe2x80x94allows for a lifting platform being lifted and lowered. The lifting platform includes carriers with which the lifting cylinder unit is connected such that the carriers are lifted when the lifting cylinder unit extends, and such that the carriers are lowered when the lifting cylinder unit moves in the opposite direction.
More particularly, the present invention relates to a lifting cylinder unit as it is used in lifting platforms for automobiles in garages.
Lifting cylinder units having a piston design are known in the art.
A known lifting cylinder unit includes a stationary rod tube having a longitudinal axis. The rod tube in one of its axial end regions is fixedly and sealingly connected to a rod head. A connecting bore is arranged in the rod head. The lifting cylinder unit is supplied with oil via the connecting bore. A cylinder tube is arranged outside the rod tube to be movable with respect to the rod tube in the direction of the longitudinal axis. The cylinder tube in one of its axial end regions is fixedly and sealingly connected to a cylinder bottom. The rod tube in its axial end facing away from the rod head is fixedly and sealingly connected to a piston. The piston includes a centrally located bore allowing for the passage of the hydraulic medium. A dynamic seal and at least one guide element are arranged at the circumference of the piston, and they contact the inner surface of the cylinder tube. The outer diameter of the piston is slightly less than the inner diameter of the cylinder tube at which the running surface for the dynamic seal is located. The cylinder tube at its axial end facing away from the cylinder bottom is fixedly connected to a guide bush. The guide bush with its inner surface is located on the outer surface of the rod tube. Finally, a supporting ring is welded to the outer surface of the cylinder tube. The supporting ring includes a surface for the application of a force to engage the carriers of the lifting platform.
To lift the lifting platform with the known lifting cylinder unit, a pump pumps oil through the connecting bore into the interior of a pressure chamber being formed by the rod tube. The oil fills the pressure chamber, and it flows through the centrically located bore of the piston in the direction towards the cylinder bottom. While the rod head, the rod tube and the piston are stationary, the cylinder bottom, the cylinder tube, the guide bush and the supporting ring are fixedly interconnected, and they are commonly movable in the direction of the longitudinal axis. When the oil pressure prevailing in the pressure chamber increases, the cylinder bottom moves away from the steady piston. The pressure chamber is sealed by the dynamic seal being located at the piston. To attain a sufficient sealing effect between the dynamic seal and the running surface or the bearing surface, the inner surface of the cylinder tube has an improved surface quality being produced by a special process. For example, the inner surface of the cylinder tube is being peeled and rumbled. Consequently, the process of producing the inner surface of the cylinder tube is rather complex.
When the known lifting cylinder unit is actuated, the guide bush with its inner surface slides over the outer surface of the rod tube. Thus, it is necessary to realize a certain surface quality of the outer surface of the rod tube within defined limits.
In the known lifting cylinder unit, no oil is located in the radial region between the inner surface of the cylinder tube and the outer surface of the rod tube and in the axial region between the piston and the guide bush. This region is connected to the atmosphere by an aerating opening extending through the cylinder tube in a radial direction. Due to the fact that certain humidity is contained in the environmental air, the inner surface of the cylinder tube and the outer surface of the rod tube have to be produced with a special process to prevent corrosion. To prevent corrosion, the inner surface of the cylinder tube and the outer surface of the rod tube include chromium plating. Due to the comparatively small outer diameter of the rod tube, the known lifting cylinder unit has little stiffness with the potential danger of folding.
Briefly described, the present invention provides a lifting cylinder unit for a lifting platform. The lifting cylinder unit has a plunger design, and it includes a stationary rod tube having a longitudinal axis, two axial end regions and an outer surface. A rod head is arranged in one of the axial end regions of the rod tube, the rod head being fixedly connected to the rod tube and including a connecting bore. A cylinder tube having two axial end regions and an outer surface is arranged outside the rod tube. The cylinder tube is designed and arranged to be movable with respect to the rod tube in the direction of the longitudinal axis of the rod tube. A cylinder bottom is arranged in one of the axial end regions of the cylinder tube, and it is fixedly connected to the cylinder tube. A guide bush having an inner surface and including a surface for the application of a force is fixedly connected to the outer surface of the cylinder tube. A pressure chamber is arranged inside the rod tube, and it is designed and arranged to be supplied with a hydraulic medium via the connecting bore. A dynamic seal is arranged in the region of the inner surface of the guide bush, and it is designed and arranged to seal the pressure chamber. A running surface is arranged in the region of the outer surface of the rod tube, and it is designed and arranged to contact the dynamic seal.
With the novel lifting cylinder unit, the number of necessary structural elements are advantageously reduced. The novel lifting cylinder unit has a simple structural design at low producing costs. The novel lifting cylinder unit has a plunger design. A plunger design means that no piston is being used. Contrary to a plunger design, in piston design, a stationary or movable piston is being used, the piston on its outer diameter carrying a dynamic seal sealingly contacting a running surface being arranged on an inner diameter. In a plunger design, the dynamic seal is arranged on an inner diameter, while the running surface is arranged on an outer diameter having improved surface quality. A dynamic seal is to be understood as a seal having a dynamic side, a relative movement taking place between the dynamic side and an associated running surface. Preferably, the relative movement is a translational movement of either the dynamic seal moving with respect to the running surface or the running surface moving with respect to the dynamic seal. A dynamic seal also has a static side at which no relative movement takes place. In the plunger design, the static side of the dynamic seal is arranged at an inner diameter to be stationary, whereas the dynamic side of the dynamic seal dynamically and sealingly contacts a running surface being located on an outer diameter having improved surface quality.
With the novel lifting cylinder unit, the guide bush is designed and arranged to transmit forces onto the carriers of the lifting platform as well as to seal of the pressure chamber. The dynamic seal is integrated into the guide bush in a way that the running surface for the dynamic seal is formed by the outer surface of the rod tube.
The outer surface of the rod tube contacts the hydraulic medium, preferably oil, up to the dynamic seal. Consequently, there is no danger of corrosion in this region being covered by the hydraulic medium. The inner surface of the cylinder tube does not require a special surface quality or special treatment since this region is not contacted by a dynamic seal. The inner surface of the cylinder tube is always covered with hydraulic medium. For this reason, no special treatment or processing and no improved surface quality is necessary, and there is no danger of corrosion.
In the novel lifting cylinder unit, the number of components requiring an improved surface quality is reduced. The rod tube is the only component requiring a ground surface. Even in the case of this one ground surface, it is advantageous that the surface to be ground is not an inner surface, but instead the outer surface of the rod tube. The treatment of a surface of an inner diameter is always more complex and more expensive than the treatment of the surface of an outer diameter.
Due to the increased outer diameter of the rod tube, the novel lifting cylinder unit has an improved stiffness minimizing the danger of folding.
The guide bush may have a first inner diameter being less than the outer diameter of the cylinder tube. Additionally, the guide bush may have a second inner diameter corresponding to the outer diameter of the cylinder tube. The guide bush carries the dynamic seal being located in the region of the first inner diameter of the guide bush in a way that the dynamic seal together with the outer surface of the rod tube seals the pressure chamber to be pressure-tight. With this arrangement, it is ensured that no hydraulic medium flows through this sealing portion. The guide bush in the region of its second inner diameter is fixedly and sealingly connected to the outer surface of the cylinder tube. For this purpose, the guide bush in the region of its second inner diameter may include an inner thread, and the cylinder tube in the portion of its outer surface contacting the guide bush may include an outer thread. A static seal is arranged in this place to prevent the hydraulic medium from exiting the lifting cylinder unit towards the environment through the connecting portion being formed between the guide bush and the cylinder tube.
Outside the pressure chamber, in an axially outward portion at the end of the guide bush facing away from the cylinder tube, a dust seal including a wiper ring may be arranged in the region of the inner surface of the guide bush adjacent to the to dynamic seal. The dust seal does not fulfill the function of sealing the pressure chamber against the exit of oil, but instead exclusively the function of sealing against the penetration of dirt and dust coming from the environment into the region of the dynamic seal. Thus, the dust seal ensures the correct function of the dynamic seal.
The surface for the application of a force of the guide bush could also be called a support surface. A shoulder of the guide bush may form it. Preferably, the shoulder is formed by the front surface of the guide bush facing the cylinder bottom. The shoulder of the guide bush contacts the carriers of the lifting platform, and it serves to transmit the lifting motion and the lowering motion, respectively, of the cylinder unit onto the lifting platform.
The guide bush may include a first guide element. For this purpose, it is possible to design the guide bush as a cast component. Thus, the inner surface of the guide bush contacts the outer surface and the outer circumference, respectively, of the rod tube to guide the cylinder tube and the guide bush being fixedly connected thereto. Due to the relatively great amount of graphite in the cast material of the guide bush, a tribological system is attained between the inner surface of the guide bush and the outer surface of the rod tube. Consequently, no scuffing occurs. It is also possible that another guide element, for example a guide strip, is arranged in the region of the inner surface of the guide bush. For example, the guide strip may be made of plastic material, and it serves to guide the guide bush and the cylinder tube in a radial direction during their translational movement with respect to the stationary rod tube along the longitudinal axis.
A second guide element may be arranged at the outer surface of the rod tube. The second guide element may also be a guide strip. The guide element is designed to be pervious in a way that the hydraulic medium is not substantially hindered from flowing through and across, respectively, the guide element. Preferably, the second guide element is arranged in the axial end portion of the rod tube facing away from the rod head. With this arrangement, the maximum possible distance between the locations of support being formed by the two guide elements is realized. The stability of the lifting cylinder unit is improved. Even when the lifting cylinder unit is extended to its maximum, the guide bush and the cylinder tube are supported by both guide elements.
A stop element may be arranged at the outer surface of the rod tube in the axial end region of the rod tube facing away from the rod head, the stop element being designed and arranged to limit the stroke of the cylinder tube. With this arrangement, the lifting cylinder unit is prevented from being further extended beyond its maximum desired position. For example, the stop element may be designed as a ring engaging a channel being located in the region of the outer surface of the rod tube. In the maximum extended position of the lifting cylinder unit, the ring contacts a shoulder of the guide bush in a way to prevent further translational movement of the guide bush in this direction.
The guide bush being designed as a cast component may include two front surfaces and a plurality of assembly openings being made by casting and being located at one of the front surfaces, the assembly openings being designed and arranged to be engaged by an assembly tool. The guide bush includes an inner thread corresponding to the outer thread of the cylinder tube. With the assembly openings and an assembly tool engaging the assembly openings, the guide bush may be screwed onto the outer surface of the cylinder tube.
The connecting bore and a check valve may be arranged in the region of the rod head. Depending on the sense of rotation of a pump conveying the hydraulic medium, the hydraulic medium, preferably oil, enters or exits the lifting cylinder unit via the connecting bore, a conduit being connected to the connecting bore and the pump. The pump provides for the necessary pressure difference to actuate the lifting cylinder unit. The check valve ensures that the maximum wanted lowering speed of the lifting cylinder unit is not exceeded to prevent uncontrolled lowering of the lifting cylinder unit possibly endangering the operator of the lifting cylinder unit.
Instead of the check valve, a control valve may be arranged in the region of the rod head. The control valve may be a mechanical 2/2-ways control valve. The 2/2-ways control valve has a passage position and a throttling position. Instead, a 2/3-ways proportional valve having two connections and three positions may be arranged in the region of the rod head. The 2/3-ways proportional valve has a passage position, a throttling position and a locking position.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and the detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims.